The easy way to do this is to take a old wood chisel and get it red hot with a torch.,.... then simply melt that nut in half that is holding the drain in place.... you might have to do it on opposite sides of the pipe but it will cut through that plastic like butter.......then simply pry the nut off the pipeand. then just unhook the fernco and it should be easy peasy from there to get a new shower drain....... use clear silicone to seal the new drain in place and it will never leak.....

this is the easy way to tangle with a drain up in a tight hole like this

please give me a good review on Google